In planographic printing, as the term implies, both the image and non-image areas lie substantially in the same plane. The non-image areas are made ink-repellent so that when ink is applied by roller to the plate surface, only the image areas accept the ink for transfer to a copy sheet. Lithography is the best known form of planography and has heretofore been the only known practical and successful process of planographic printing. It works on the theory that water and oil are immiscible. The non-image areas are made water-receptive (hydrophilic), and when water-wet they repel the oily ink. The image areas are ink-receptive (organophilic) and water-repellent (hydrophobic). On the press, the plate conventionally is first dampened with a fountain solution (which wets the background or non-image areas), after which ink is rolled over the plate by form rollers. The ink coats the image areas, but is repelled from the dampened non-image areas.
In present times, most lithography is done by "offset", wherein the plate does not itself contact the copy sheet, but instead contacts a rubber-like blanket roller, or cylinder during each revolution. The blanket receives the ink image and, in turn, revolves and transfers ("offsets") it to the copy sheet.
Lithography was well established as a separate branch of the printing industry in the early 1900's. Because a number of advantages are realized by the planographic method of printing, lithographic printing has been a well-recognized and widely-accepted part of the printing art for many years. Despite numerous developments and improvements which have been made over the years, and notwithstanding difficulties inherent therein, lithography has retained its total reliance on the original concept that water which is coated over the non-image areas of a printing surface, being immiscible with oil, will reject an oil-based printing ink.
One of the difficulties inherent in having both ink and fountain, or dampening, solutions present is that the dampening solution applied to the plates flows back into the train of inking, or form rollers on the press, during the course of the printing run, causing emulsification of the ink. In addition to back flowing, the fountain solution also tends to flow forward over the offset or blanket cylinder, moistening the paper causing it to curl and change dimension. This creates special difficulties in securing accurate registration in color printing where the paper undergoes multiple passes through the presses. Control of the delicate balance between ink and fountain solution, which is necessary to produce image fidelity and uniformity, is difficult to maintain, and must be constantly watched, especially as conditions change on the press during the course of a printing run. This is further complicated by the difficulty in maintaining the desired chemical consistency of the fountain solution on the press, especially in long press runs. The fountain solution is generally a mixture of water, acid, buffer and gum designed to keep the non-image areas of the plate passivated and ink repellent to prevent them from receiving ink. The nature of the fountain solution affects the working properties of the ink being used. Too much acid causes image removal (sharpening), retards drying and causes roller stripping. Too much gum encourages emulsification. The lithographic process requires that a balance be maintained between ink and fountain solution. If the water layer on the plate falls below a certain point, the ink begins to catch up into the non-image areas and scumming, or sensitization of the non-image areas, occurs. Scumming can be eliminated by increasing the amount of water reaching the plate or by lowering the temperature of the fountain solution. This latter change increases the viscosity of the ink on the plate and form rollers making it less mobile and thereby less prone to water pick-up and mechanical breakdown into the non-image areas. In an effort to minimize these problems, the proper formulation of fountain solution itself has become a highly complex and demanding art since the fountain solution in many cases determines the level of artistic and commercially acceptable quality which can be attained in lithographic printing.
During the more than half-century in which lithography has been an established commercial form of printing, in which the desirability of eliminating the step of dampening the plate with water has been recognized, no one was able to develop a successful planographic printing plate having a printing surface with background areas that are ink-repellent without being pre-wet by an ink-immiscible liquid until the recent advent of Driography. Driography provides for the first time a substantially simplified planographic printing process wherein need for all the sophisticated dampening systems has been obviated.
A Driography printing plate and process are described in U.S. Pat. No. 3,511,178 to J. L. Curtin, herein incorporated by reference. The Driography process is based on the properties of the plate involving adhesion. In general, Driography comprises a printing plate having a background surface, or non-image area with a sufficiently low adhesion to the printing ink so that, without pre-wetting the plate, the ink that is applied to the plate in such areas will not split away and transfer from the inking rollers to the plate. That is, the adhesion of the ink to the inking roller and the cohesion forces between the ink particles are both greater than the adhesion between the ink and the surface of the plate and the ink will not transfer to the surface. Although when a press utilizing Driography is started up, the ink spreads over the entire printing plate, the non-image areas of the plate are rapidly cleaned by the form rollers since the forces between the roller and the ink are greater than the forces between the ink and non-image areas.
Driography has many advantages over conventional lithography. The printing quality and cosmetic appeal of the final product are better than conventional lithography utilizing a wet offset process. The press can operate at faster speeds. The makeready is shorter and easier, since there is no need to achieve a balance between the ink and fountain solution or the solution and the printing plate, each of which balances, in conventional, wet lithographic processes, requires a considerable amount of time to achieve, as well as constant monitoring to maintain them for successful printing. Furthermore, the absence of the fountain solution system makes the process more economical.
Although Driography has these and other advantages, it results in the provision of new problems which require resolution if the apparent advantages provided by Driography are to be utilizable in a practical manner. An evaluation of planographic inks can be divided into three categories, (1) those which evaluate the handling characteristics of the ink under ambient conditions; (2) those which relate to how the ink will perform on a press; and (3) those which relate to the quality (e.g., the physical, optical and chemical properties) of the printing produced.
An example of the new problems created involves the absence of the fountain solution system which results in the press heating up and the printing plate reaching higher temperatures than in conventional, wet lithography where approximately 40% of the water of the fountain solution evaporates from the press and in doing so, cools the rollers and the printing plate. In a conventional, wet lithographic process, the press will normally run at temperatures up to about 90.degree. to 100.degree. F., depending upon ambient conditions, air-conditioning in the press room, etc., and the printing plate to which the ink is applied will typically be about 10.degree. F. below this press temperature. In Driography, the printing press will normally be about 10.degree. F., or more, higher than the wet press temperature due to the absence of the fountain solution. Because of this extra heat in Driography, and the loss of the acid present in the fountain solution which immobilizes part of the ink drying system to provide a retarding effect upon ink drying, conventional lithographic inks tends to dry on the press faster in Driography than in conventional, wet lithography. For example, a 10.degree. C. rise in press temperature in general doubles the ink drying rate so that the ink dries on the press in approximately one-half the time. Since the temperatures in the press are approximately 20.degree. F., higher in Driography than in wet lithography, conventional lithographic inks tend to dry on the press when used in Driography and do not move through the press properly to transfer to the copy sheet.
Press stability is, therefore, not as good with Driography as it is with conventional wet lithography. By press stability is meant the ability of the ink, under press-operating conditions (e.g., temperatures and shear rates), to move through the press (e.g., through the rollers, plate and blanket and transfer to and from the printing plate) and the tendency of the ink not to dry in the roller train, plate and blanket. The application and form rollers in the printing process both supply ink and keep the printing plate clean by removing ink from the non-image areas of the plate. An ink has low press stability when the ink dries in the roller train, on the printing plate or blanket, or if the ink fails to transfer from the form rollers to the printing plate.
Toning is another example of a problem with Driography. Toning occurs when ink is not refused by the non-image areas of the printing plate. The result of toning is the application of ink to the copy sheet in the non-image areas. Toning may be in the form of a light background color in the non-image areas of the copy sheet so that the copy sheet appears dirty. In severe toning, the background, or non-image areas of the copy sheet, merge into the printed, or image, areas.
Toning can be related to temperature. The toning temperature of an ink is that temperature at which the ink, under given shear rates, loses internal cohesion and becomes pseudoplastic in behavior. At this temperature, control of ink application, i.e., ink being picked up from the printing plate by the form rollers, is lost. Toning is, accordingly, analogous to scumming in conventional, wet lithography. Conventional lithographic inks have very low toning temperatures when used in Driography, working well in Driography at about 75.degree. F., and below. Although some conventional lithographic inks will function at slightly higher temperatures, e.g., up to 85.degree. F., their utility is limited by uncertainty of operation and limitations on color.
Conventional planographic inks do not perform well in Driography printing. They are not press-stable, particularly under the high temperatures and shear rates occurring in Driography printing, which does not have the advantages provided by a fountain solution. In addition, toning is produced and the quality of the printing itself is less than that desired.
It is considered that at low shear rates, typically the Driography ink system should be near Newtonian and at high shear rates, it should move into the dilatant zone. This accounts for the plate/form roller behavior to give a wipe action of the form roller over the plate with the ink tending to break clean from the low energy surface. In this type of system, the faster the roller movement, the more resistant to movement there is from the system. In addition, the Driography ink system tends to heat up and is, therefore, temperature sensitive.